1. Field of the Invention
The present invention relates to a light beam scanning apparatus which scans, on a photosensitive drum, a light beam based on image data. The present invention also relates to an image forming apparatus to which the light beam scanning apparatus is applied.
2. Description of the Related Art
A laser driving circuit in an image forming apparatus supplies a predetermined DC current (bias current) to a laser and, in addition to this current supply, supplies even a switch current that is switched in accordance with image data, thereby causing the laser to emit a light beam. As a characteristic of a laser, its light emission amount changes in proportion to a supplied current. Hence, when the current to be supplied to the laser is controlled, the laser emission amount for image formation can be controlled.
As control to keep a predetermined laser power, APC (Auto Power Control) is known. In APC, the light emission amount of a laser is detected. The light emission amount detection level is compared with a reference value as the target value of laser power. Accordingly, the current amount to be supplied to the laser is controlled to maintain a predetermined laser power.
A light beam emitted from a laser is reflected by a polygon mirror rotated at a predetermined rotational speed by a polygon motor and scans the surface of a photosensitive drum. That is, an electrostatic latent image is formed on the photosensitive drum by scanning the light beam whose light emission timing is controlled in correspondence with the image data.
Conventional APC is started after the polygon motor reaches a predetermined rotational speed, and the rotational speed stabilizes. A predetermined time is necessary until the polygon motor reaches a predetermined rotational speed, and the rotational speed stabilizes. For example, when the polygon motor that is set in a stopped state in a power saving mode or the like is reactivated, some standby time is required until the start of APC. As a result, the standby time from reactivation to image formation is long.